The invention relates to a process for preparing solid melamine, by depressurizing a dispersion of ammonia and liquid melamine, whereupon solid melamine precipitates.
Melamine is preferably prepared by pyrolyzing urea, using either low-pressure processes or high-pressure processes, for example those described in xe2x80x9cUllmann""s Encyclopedia of Industrial Chemistry, Vol A 16, 5th ed (1990), pp. 171-185xe2x80x9d. Depending on the preparation process, the melamine synthesized comprises from about 94 to 98% by weight of melamine, and also in particular melam, melem, ureidomelamine, ammeline, and ammelide as significant impurities, and has to be further purified through particular steps of the process for the more demanding application sectors. To obtain solid melamine, the liquid melamine melt may be cooled, for example using water, using aqueous melamine-containing solutions or suspensions, or using cold inert solids or solid melamine, as in AT 159/98, for example in a fluidized bed. A particularly advantageous method is to inject and depressurize an ammonia-containing melamine melt, for example as in WO97/20826, into a cooling vessel, in which an atmosphere of ammonia is present, whereupon pure solid melamine precipitates. However, this process does not give ideal results under all conditions of pressure and temperature.
The object was therefore to find a process which, irrespective of the temperature used and of the pressure used, gives solid melamine of good quality via depressurization, and in a wide range of temperature and of pressure, and in particular at low melt pressures.
It has now been found that this object can be achieved in that the melamine melt to be depressurized comprises excess ammonia in addition to the dissolved ammonia, giving a two-phase mixture in the form of a dispersion of ammonia and liquid melamine.
The invention therefore provides a process for preparing solid melamine by depressurizing liquid, ammonia-containing melamine, characterized in that
a) the liquid ammonia-containing melamine is mixed with excess ammonia, whereupon a dispersion of ammonia and liquid melamine forms,
b) where appropriate, the dispersion is aged under the pressure generated by ammonia,
c) the dispersion is depressurized, whereupon solid melamine precipitates,
d) where appropriate, the solid melamine is aged under the pressure generated by ammonia,
e) and then, where appropriate and in any desired sequence, there is further depressurization to atmospheric pressure, and cooling to room temperature, and the melamine is isolated.
The ammonia (gas phase) is preferably supercritical and is preferably in finely dispersed form in the liquid melamine melt (liquid phase) producing a very finely dispersed xe2x80x9cmelamine foamxe2x80x9d. The mixing process forms a dispersion of melamine and ammonia, the liquid melamine becoming saturated with ammonia. It is preferable for the liquid melamine phase to have been saturated with ammonia. According to the invention it is possible either for the ammonia to have been dispersed in the liquid melamine or for the liquid melamine to have been dispersed in the ammonia. It is important that the entire amount of ammonia (dissolved ammonia and ammonia present in the gas phase) is sufficiently great for the amount of heat dissipated during the depressurization to be that needed to solidify the melamine. A particular advantage of the invention is therefore that with the aid of the excess ammonia dispersed in the melt it is possible to obtain sufficient dissipation of heat to solidify the melamine during the depressurization even at relatively low pressures on, and relatively high temperatures of, melamine melts where relatively little ammonia has been dissolved in the melamine melt. The amount of excess ammonia in the melamine melt depends in particular on the level of temperature of, pressure on, and ammonia saturation of the melt prior to the depressurization, and on the extent to which the melamine is to be cooled below its melting point once it has been solidified. High temperatures of, and low pressures on, the melt therefore require larger amounts of excess ammonia than temperatures which are just above the melting point of the melamine, which depends on the ammonia pressure used. On the other hand, high pressures necessitate smaller amounts of excess ammonia. The amount of excess ammonia may therefore vary within wide limits.
Since the melting point of the melamine at lower pressures after depressurization is higher than at high pressures, it is also possible according to the inventionxe2x80x94in particular if the temperature of the melt is not too far above the melting point, which depends on the pressure, and the amount of excess ammonia is not very greatxe2x80x94that the temperature during solidification remains the same or even rises.
According to the invention, it is preferable that the pressures at which the liquid ammonia-containing melamine is mixed with ammonia are from about 50 to 1000 bar, and that the liquid ammonia-containing melamine is then depressurized to a pressure of from about 1 to 200 bar, whereupon solid melamine precipitates. Depending on the procedure selected, the pressure both prior to and after the depressurization may vary over a wide range. The upper pressure limit prior to the depressurization is preferably about 600 bar, preferably about 350 bar or about 250 bar. However, the upper limit may also be about 150 bar or about 130 bar. The lower pressure limit prior to the depressurization is preferably from about 60 to 80 bar. The pressure after the depressurization may likewise vary within a wide range. If an annealing process follows immediately, depressurization takes place to relatively high pressures, otherwise depressurization to atmospheric pressure is possible. The pressure after the depressurization is therefore preferably from about 1 to 100 or 150 bar, particularly preferably from about 1 to 60 bar. However, it may also be from about 10 to 20 bar.
During the mixing with ammonia, or prior to the depressurization, the temperature of the liquid ammonia-containing melamine is preferably in the range from about 60xc2x0 C. above the melting point of the melamine, which depends on the ammonia pressure used, to just above the melting point of the melamine, which depends on the ammonia pressure used, particularly preferably at temperatures between about 1 and 40xc2x0 C., very particularly preferably between 1 and 20xc2x0 C., above the melting point of the melamine, which depends on the ammonia pressure used. The most useful temperature is only very slightly above the melting point of the melamine, which depends on the ammonia pressure used. The desired depressurization temperature is most particularly preferably below about 350xc2x0 C. It is preferable for the liquid, ammonia-containing melamine to have been saturated with ammonia.
Suitable mixing equipment can be used for mixing the liquid, ammonia-containing melamine with excess ammonia, forming a dispersion, for example mixers, stirrers, reactors with naturally aspirated stirrers, or static mixers, injectors, ejectors, or other suitable mixing equipment. The melamine melt may be mixed with either gaseous or liquid ammonia, but if liquid ammonia is used the melt must not solidify. The temperature of the melt preferably lowers during this process and is thus brought into the vicinity of the desired temperature, or to the desired temperature.
Prior to the depressurization, either the pressure or the temperature may, where appropriate, be increased, lowered or held constant, using any desired method, either prior to or else after the mixing of melamine with ammonia, but the melt must not solidify during this process. To achieve a particularly good quality of melamine, it is advantageous for the liquid melamine melt to be aged under the pressure generated by ammonia prior to the depressurization, either prior to or after the mixing of the liquid melamine and ammonia. This preferably takes place for from about 1 min to 10 h depending on the process conditions selected in the temperature range from about 350xc2x0 C. to just above the melting point of the melamine, which depends on the ammonia pressure used, and preferably at temperatures above the melting point of the melamine, which depends on the ammonia pressure used, by from about 1 to 60xc2x0 C., particularly preferably by from about 1 to 40xc2x0 C., more particularly preferably by from about 1 to 20xc2x0 C. It is advantageous here to lower the temperature of the liquid melamine, for example by introducing liquid or gaseous ammonia. The pressure during the aging process here is in the range from about 50 to 1000 bar, preferably from about 80 to 600 bar, particularly preferably from about 130 to 400 bar. If the aging takes place after the mixing of melamine and ammonia, care must be taken that the dispersion is retained during the aging process.
The temperature at which the dispersion of ammonia and liquid melamine is depressurized is preferably above the melting point of the melamine, which depends on the ammonia pressure used, by from about 1 to 60xc2x0 C., particularly preferably from about 1 to 40xc2x0 C., more particularly preferably by from about 1 to 20xc2x0 C., whereupon the solid melamine deposits, its melting point now being higher at lower pressure.